Hello RX400h owners. I have a 2006 RX400h with 116k miles on it. It is in very good cosmetic condition and I love it!
I recently had this issue - vehicle shuts down while driving but then restarts and drives perfectly. Dash lights up with the "Check Hybrid System" warnings. The intermittent nature of the problem made it "feel" like it was an electrical short or something. Of course it is scary because the vehicle shuts down while driving and could cause a wreck! It means that we don't have confidence in our vehicle for long trips, etc.
I ultimately replaced the cooling fans and the relay pack inside the hybrid battery. It seems to have fixed the issue. I have been driving the vehicle for a couple of months now and have had no recurrence of the issue.
I did the repair myself, so I thought it would be useful to post a description of the DIY. It was actually very easy to do. It is a complex job - there are lots of steps - but each step is easy. If you take your time and make sure you put all of your parts in an organized pile, it is easy to put the vehicle back together.
I think that there are many shops out there - including the dealerships - that would be more likely to tell you that you need to replace the entire hybrid battery for $2200 (used/reconditioned) or $4000 (new), or more. This is a good DIY repair. I would go so far as to say that it might be a good preventative measure to take once your 400h gets to be older with higher mileage.
First of all, I agree that the first step is to get your vehicle checked for any recalls. Mine had the inverter recall done by a previous owner, so that was not a problem. I also agree with the idea that if the 12v battery is old, replacing it is not a bad idea - they do seem to be undersized and really only last about 3 years in these vehicles. When they get old they can contribute to electrical issues.
So, I had a similar experience to RABeyer where a local mechanic had scanned the system and found "Lost connection to hybrid battery" message. There's a simple ECU inside the hybrid battery that "talks" to the main ECU under the dash. When those two lose connection, the hybrid system shuts down. Then we limp our vehicle over to the side of the road, turn the key off and turn it back on and, presto!, the car runs perfectly again. That's because the communication issue between the two ECUs is intermittent. If your vehicle has exactly this problem, then replacing your cooling fans may fix your issue - and if you do it yourself it may only cost you about $400. By the way, my understanding is that the problem is not caused by the fans failing to cool the batteries, but is caused by the fans getting old and causing electrical "noise" in the system due to electrical resistance in the fan motor.
These video is long but it explains the issue in detail:
So, the repair involves:
1. Remove back seats.
2. Open hybrid battery pack.
3. Remove and replace the three cooling fans.
4. Close hybrid battery pack.
5. Replace back seats.
I used these videos to do this repair:
and
So I attached some pictures. Battery Pack shows what the battery pack looks like when you take out the seats and lift the carpet.
Back seat shows what it looks like when you take the lid off of the battery pack.
Organized shows how I had my car parts laid out in my garage.
3 Fans shows the fans that you will replace. They are the three snail-shell shaped items along the back of the battery pack (along the left in the picture). They simply unplug from the harness and are held in by 3 screws each. Really simple remove/replace job.
Relay Pack shows the high voltage relay pack inside the hybrid battery. I went ahead and replaced this, as well. It looked like it was in good shape, but relays can wear out internally. I figured that I would go ahead and replace this while I had the car taken apart. I don't think this was actually necessary to fix the issue, but it only cost about $400, so I just figured it was a good preventive repair. This was also easy to swap, just unplug the connectors, remove it by taking out 3 bolts, plug in the new unit, and bolt in the new unit.
Parts is just the boxes that I received from Lexus. I was able to buy brand new parts from the dealership and have them shipped to my house. Since you are replacing electrical components, I think it only makes sense to use brand new parts. If you put in rebuilt or used, you may be just replacing bad with bad. The fans were part number G923048010 and were $110 each. The relay pack was part number G384048010 and was $330.
So, if this is your problem, this is an easy fix. I did not need any specialty tools. I mostly used a simple ratchet and sockets. I did have an electrical tester and I did follow the instructions in the videos to make sure the high voltage system was safe to work with.